Method for processing a digital image, device, terminal equipment and associated computer program

ABSTRACT

A method for processing a digital image including image elements having a first luminance component. The first luminance component has a first value in a first predetermined interval. The display can render second values of second luminance components included in a second predetermined interval. The method includes: determining information representative of image brightness perceived by an observer, based on the first values of the first luminance component; calculating an expansion exponent as a function of the determined brightness information; transforming the first luminance components into the second luminance components, including calculating an intermediate luminance value by applying the calculated expansion exponent to the first luminance component value, multiplying the intermediate value by a length of the second interval, and obtaining the second value of the second luminance component based on the multiplied intermediate luminance value. The expansion exponent is based on a logarithm, of the determined brightness information.

1. CROSS-REFERENCE TO RELATED APPLICATIONS

This Application Continuation-in-Part of U.S. application Ser. No.16/063,441, filed Jun. 18, 2018, which is a Section 371 National StageApplication of International Application No. PCT/FR2016/053321, filedDec. 9, 2016, the content of which is incorporated herein by referencein its entirety, and published as WO 2017/103399 on Jun. 22, 2017, notin English.

2. FIELD OF THE INVENTION

The field of the invention is that of digital image processing anddigital image sequences whose colour information is represented in afirst range of values, with a view to rendering them on a display devicecapable of handling them to represent on a second range of values,superior to the former.

The invention may especially, but not exclusively, apply to theconversion of colour intensities of digital images represented in astandard or SDR format (for “Standard Dynamic Range”) to be rendered ona display device conforming to an HDR format (for “High Dynamic Range”).

3. PRESENTATION OF THE PRIOR ART

A new generation of audio-visual content reproduction devices can beseen today, such as televisions, so-called HDR which are adapted todisplay images with a wide range of colour intensities. These screensoffer a high peak level and increased levels of contrast between lightand dark areas of the image, which gives the user unparalleled proximityto reality.

Currently, this technology still coexists with the SDR format, whichremains the reference for the transmission of audio-visual content, sothat to take advantage of the increased capabilities of an HDR screen,it is necessary to convert the SDR content received in HDR format beforedisplay.

The article by Akyuz et al, entitled «Do HDR displays support LDRcontent? A Psychophysical Evaluation», published by ACM SIGGRAPH 2007Papers, page 38, in 2007, discloses a method for expanding the colourintensities of an input digital image, based on a simple linearoperator. It comprises calculating the luminance component of the outputimage Y₂ as a simple linear function of the luminance component Y₁ ofthe input image, according to a formula of the type:

$\begin{matrix}{L_{2{({x,y})}} = {L_{\max} \cdot \frac{{Y_{1}\left( {x,y} \right)} - Y_{1,\min}}{Y_{1,\max} - Y_{1,\min}}}} & (1)\end{matrix}$where (x, y) are the coordinates of a image element in the input image,Y_(1,max) the maximum value taken by the luminance component in theinput image and Y_(1,min) its minimal value.

The subjective results obtained by Akyuz with this operator areconsidered as the best in the literature for normally exposed images.

The article by Masia et al, entitled «Evaluation of Reverse Tone MappingThrough Varying Exposure Conditions» published in the journal «ACMTransactions on Graphics», published by ACM, volume 28, page 160, in2009, discloses a method for expanding colour intensities of a digitalimage. It consists in particular in applying a non-linear globaloperator of intensity expansion, to the luminosity information of theelements of the input image. This operator takes the form of an exponentexpressed as an affine function of an indicator representative of theimage, so-called “image key”.

This key indicator is well known to those skilled in the art and isexpressed as follows

$\begin{matrix}{k = \frac{{\log\; Y_{moy}} - {\log\; Y_{\min}}}{{\log\; Y_{\max}} - {\log\; Y_{\min}}}} & (2)\end{matrix}$

where

${{\log\; Y_{moy}} = \frac{\left( {\sum\limits_{x,y}{\log\left( {{Y\left( {x,y} \right)} + \delta} \right)}} \right)}{n}},$n the number of elements in the image, Y(x,y) is the luminance intensityof an element of the image and δ is a positive real number that takes asmall value to avoid singularities when the intensity of a pixel iszero.

The logarithm of luminance is indeed known as a good approximation of aluminosity or illumination level perceived by the human visual system.The image key k thus provides an indication of a global luminosity ofthe image, as perceived by an observer.

The global operator for expanding the luminosity intensities of theinput image takes the following form:γ=a·k+b  (3)

where a is a real number that is 10.44 and b is a real number with avalue of −6.282.

On the set of tested images presented in the article, we can see thatthe value of the operator γ increases with the value of the image key,the extreme values being equal to 1.1 and 2.26.

An advantage of this solution is that it makes it possible to improvethe perceived quality of the images in a simple and realisable way inreal time. In particular, it gives good results on exposed images havinga sufficient level of contrast.

4. DISADVANTAGES OF THE PRIOR ART

A disadvantage of the methods described in the prior art is that theyare not suited for all types of images. In particular, for images havingmore extreme contrast and luminosity levels than the tested imagestested, they confer the processed images an unsightly artificialappearance, which is hardly faithful to the input image.

5. SUMMARY OF THE INVENTION

An exemplary embodiment of the present invention relates to a methodcomprising the following acts performed by a processing device:

-   -   receiving at least one digital image, said image comprising        image elements, an image element being associated with colour        information represented in a first colour space comprising a        first luminance component said first luminance component having        a first value within a first range of predetermined values;    -   processing the at least one digital image for rendering on a        display device, said display device being adapted to restore        second values of second luminance components of the image        elements included in a second range of predetermined values,        wherein the processing comprises:    -   determining an information representative of a global luminosity        level of the image perceived by an observer, based on the first        values of the first luminance component of the elements of the        image;    -   calculating an expansion exponent as a function of the global        luminosity level information determined;    -   transforming the first luminance components of the image        elements into said second luminance components, comprising for        an element of the image, the calculation of an intermediate        luminance value by applying the expansion exponent calculated to        the first value of the first luminance component, the        multiplication of the calculated intermediate value by a length        of the second range of predetermined luminance values, and        obtaining the second value of the second luminance component        based on the multiplied intermediate luminance value,

The method according to the invention is remarkable in that thecalculated expansion exponent is a decreasing function of the determinedglobal luminosity level information, based on the logarithm of saiddetermined global luminosity level information.

Thus, the invention offers a new and inventive solution for expandingthe range of values of the luminance information to adapt the format ofan input image to that of the display device whose range is wider.

Unlike the prior art, which chooses an expansion exponent whose valueincreases with the global luminosity level of the image, the inventionoffers an expansion exponent whose value decreases when the globalluminosity level increases. Moreover the use of the logarithm functionallows viewers to get a better visual rendering.

The inventors have identified five classes of image stylesrepresentative of the different possible combinations of luminosity andcontrast levels of a large set of test sequences. They then implementedan experiment in which they applied different correcting exponent valuesto the image sequences of each of these style classes, then asked theobserver panel to evaluate its perceived quality.

From the results obtained, the inventors have found on the one hand thata particular corrective exponent value could be associated with each ofthe classes. On the other hand, they have established a simplemathematical relation between the global luminosity level of the imagesof a class and the corrective exponent value to be applied to the imagesof this class allowing to obtain an output image, adapted from aperceptual point of view.

According to the invention, the calculated exponent is proportional tothe inverse of the logarithm of the information representative of aglobal luminosity level of the image.

An advantage of this mathematical relation is that it allows a faithfulrendering of the lighting style of the image while remaining simple toimplement with limited computing resources compatible with real-timeprocessing requirements. Another advantage is that the exponent isadapted to an optical—electrical tone curve, which can be used prior tothe step of determining an information representative of a globalluminosity as explained below.

According to an advantageous characteristic of the invention, the stepof determining a global luminosity level comprises obtaining a medianvalue of the luminance component of the image, in that the informationrepresentative of a global luminosity level of the image is proportionalto the obtained median value.

An advantage of using the median luminance values of an image is that ittakes values that remain stable from one image to another of a sequenceof images. This avoids any flickering or fluttering effect during therendering of the sequence.

Alternatively, the mean value of the luminance component can beobtained, implying a simplification of calculations.

According to another aspect of the invention, when the first range ofpredetermined values is of length greater than the second range, thetransforming act implements the following equations:

$Y_{2} = \left( \frac{Y_{1}}{L_{\max}} \right)^{\frac{1}{\gamma}}$${{with}\mspace{14mu}\gamma} = \frac{1}{{{gain} \times {\log_{10}\left( Y_{med} \right)}} + {offset}}$where Y1 designates the first luminance component, Y2 the secondluminance component, L_(max) the length of the second range of luminancevalues, log₁₀ the decimal logarithm, γ the expansion exponent applied tothe first luminance component Y1, Y_(med) the median luminance value,gain a predetermined gain value and offset a predetermined offset value.According to another aspect of the invention, when the second range ofpredetermined values is of length greater than the first range, thetransforming act implements the following equations:

Y₂ = L_(max) ⋅ Y₁^(γ)${{with}\mspace{14mu}\gamma} = \frac{1}{{{gain} \times {\log_{10}\left( Y_{med} \right)}} + {offset}}$where Y1 designates the first luminance component, Y2 the secondluminance component, L_(max) the length of the second range of luminancevalues, log₁₀ the decimal logarithm, γ the expansion exponent applied tothe first luminance component Y1, Y_(med) the median luminance valuewhich is normalised and clipped, gain a predetermined gain value andoffset a predetermined offset value.

An advantage of this mathematical expression that links the secondluminance component to the first is that it is simple to implement,while ensuring a realistic and respectful rendering of the originallighting style of the input image, regardless of the global luminositylevel of the input image.

According to yet another aspect of the invention, the first colour spacefurther comprises first chrominance components, said method furthercomprising a step of transforming the first chrominance components ofthe image into second components, by applying to the first chrominancecomponents an expansion coefficient proportional to a ratio between thesecond luminance component and the first luminance component, accordingto the following expression:

$C_{2} = \frac{{C_{1} \cdot Y}\; 2}{Y_{1}}$

An advantage of this embodiment is its simplicity.

According to another aspect of the invention, the first colour spacefurther comprises first chrominance components, said method furthercomprising a step of transforming the first chrominance components ofthe image into second chrominance components, comprising a substep ofcolour correction by applying to the first chrominance components acorrection function which depends on the first and the second luminancecomponents and a saturation factor, which is a reality strictly greaterthan 1, according to the following expression:

$C_{2} = {\left( {{\left( {\frac{C_{1}}{Y_{1}} - 1} \right) \cdot s} + 1} \right)Y_{2}}$

An advantage of this embodiment is that by saturating the chrominancecomponents, it allows for more intense colour rendering.

Advantageously, the step of transforming the first chrominancecomponents comprises a sub-step of converting a first colour space to asecond colour space, larger than the first.

One advantage is to avoid the truncation of colour intensities and thusthe appearance of defects on the output image.

As an alternative, the step of transforming the first chrominancecomponents comprises a sub-step of converting a first colour space to asecond colour space having the same size.

According to another aspect of the invention, the method, furthercomprising prior to the transforming act, setting the expansion exponentto a first predetermined value if the calculated value is less than saidfirst predetermined value or setting the expansion component to a secondpredetermined value if the calculated value is greater than said secondpredetermined value.

An advantage of containing the expansion exponent between thepredetermined values is that these values may represent the limits ofcontrast adjustments required to match the original lighting style ofthe input image.

The method which has just been described in its different embodiments isadvantageously implemented by a device comprising:

-   -   a reprogrammable computing machine or a dedicated computing        machine, capable of and configured to:    -   receive at least one digital image, said image comprising image        elements, an image element being associated with colour        information represented in a first colour space comprising a        first luminance component separated from chrominance components,        said first luminance component having a first value comprised in        a first predetermined value range; and    -   process the at least one digital image for rendering the image        on a display device, said display device being adapted to render        second values of second luminance component of the images        elements included in a second predetermined range of values,        wherein the processing comprises:    -   determining an information representative of a global luminosity        level of the image, perceived by an observer, based on the        values of the first luminance component of the elements of the        image;    -   calculating an expansion exponent as a function of the global        luminosity level information determined;    -   transforming the first luminance components of the image        elements into said second luminance components, comprising for        an element of the image, the calculation of an intermediate        luminance value by applying the expansion exponent calculated to        the first value of the first luminance component, the        multiplication of the intermediate value calculated by a length        of the second range of predetermined luminance values, and        obtaining the second value of the second luminance component        based on the multiplied intermediate luminance value.

Such a device is remarkable in that the calculated expansion exponent isa decreasing function of the determined global luminosity levelinformation, based on the logarithm of said determined global luminositylevel information.

Correlatively, the invention also relates to a terminal equipmentcomprising a receiver capable of and configured to receive a sequence ofdigital images via a communication network and a transmitter capable ofand configured to transmit the sequence of images to a display devicecapable of and configured to restore it, characterised in that itcomprises a device for processing at least one digital image accordingto the invention.

This terminal equipment may be a personal computer, a set-top box TV, adigital television etc.

The invention further relates to a computer program comprisinginstructions for implementing the steps of a method for processing atleast one digital image as described above, when this program isexecuted by a processor.

The invention also relates to a computer program comprising instructionsfor implementing the steps of a method for processing a digital image asdescribed above, when this program is executed by a processor.

These programs can use any programming language. They can be downloadedfrom a communication network and/or recorded on a computer-readablemedium

The invention finally relates to a processor-readable recording orstorage medium, integrated or not to the device for processing a digitalimage according to the invention, optionally removable, respectivelystoring a computer program implementing the processing method, asdescribed above.

6. LIST OF FIGURES

Other features and advantages of the invention will become evident onreading the following description of one particular embodiment of theinvention, given by way of illustrative and non-limiting example only,and with the appended drawings among which:

FIG. 1 shows schematically a chain of processing an SDR format image oran input image sequence to provide an image or a sequence of outputimages in HDR format respectively;

FIG. 2 shows schematically the steps of a method for processing adigital image according to the invention;

FIG. 3 details the step of determining information representative of aglobal luminosity level according to one embodiment of the invention;

FIGS. 4A and 4B show examples of decreasing functions of the globalluminosity level information according to two embodiments of theinvention;

FIG. 5 details the steps of calculating an expansion and derivationexponent of the colour components according to a first embodiment of theinvention;

FIG. 5A details the steps of calculating an expansion and derivationexponent of the colour components according to a second embodiment ofthe invention;

FIGS. 6A to 6D show chromaticity diagrams of the input image and of theoutput image according to the first and second embodiments of theinvention;

FIG. 7 shows examples of classes of lighting styles of input imagesdefined according to their luminosity and their contrast;

FIGS. 8A to 8E show examples of expansion exponent curves obtained bythe method for processing a digital image according to the invention forinput images belonging to predetermined classes of lighting styles;

FIG. 9 shows in a comparative manner examples of output images obtainedafter processing according to the invention and according to twosolutions of the prior art;

FIG. 10 comparatively shows the results of subjective tests carried outon a set of images with the processing method according to the inventionand two solutions of the prior art; and

FIG. 11 shows schematically an example of hardware structure of a devicefor processing a digital image according to an embodiment of theinvention.

7. DESCRIPTION OF A PARTICULAR EMBODIMENT OF THE INVENTION

As a reminder, an object of the invention is to provide a method toextend the range of colour intensities of an input image according to astandard format to render it on a display device having to a wider rangeof colour intensities. The general principle of the invention is basedon the determination of an information representative of a globalluminosity level of the image as perceived by an observer and on theapplication to the intensities of the image of an expansion exponent,expressed as a decreasing function of the global luminosity level of theimage.

In relation to FIG. 1, we consider a processing chain of an input imagesequence (II_(n)) in the SDR format, with n an integer between 0 and N,with N a non-zero integer, in order to display it in HDR format, whereluminance values are included in a greater range than luminance valuesin SDR format.

The images of the input sequence are two-dimensional (2D). Theirelements are pixels. Of course, the invention is not limited to thisexample and also applies to three-dimensional (3D) or multiview images,the elements of which are voxels.

The images of this sequence can take different spatial dimensions suchas for example SD images (for Standard Definition), HD (for HighDefinition), UHD (for Ultra High Definition), 4K, which is four timesthe definition of an HD and 8K image, which is eight times thedefinition of an HD image. The input sequence may have various framerates among the following values of 24, 25, 30, 50, 60, 120 etc. Thecolour intensities of its image elements can be encoded over a bit depthfor example equal to 8, 10, 12 or 16 bits.

It is assumed that this sequence of images has previously been obtainedeither in raw form directly at the output of an acquisition module, suchas for example a video camera, or in decompressed form at the output ofa decoder which had received it via a communication network.

For example, the input image sequence (II_(n)) is in the format R′G′B′(for “Red Green Blue”) . . . as specified in the BT.709 standard whichdefines the values of HDTV standards for the production andinternational exchange of audio-visual programs. The colour informationis expressed in three components R′, G′, B′ which each take valuesbetween 0 and 255.

Of course, the invention is not restricted to this colour space and canalso handle input images compliant with other formats such as BT.2020,BT.601, DCI-P 3, etc.

These colour information R′G′B′ correspond to a computer or electricalencoding of the colours of the image elements. An optical electricalconversion operation is performed in T1 to restore the opticalintensities of the colours of the image. The RGB optical intensitiesthus obtained take values between 0 and 1.

For instance, the optical electrical conversion may follow a curve(electrical-optical tone curve) allowing to slightly decrease orincrease the intensities of the image colours for some specific tones.

These RGB optical intensities are presented in T2 at an ITMO module (for“Reverse Tone Mapping Operator”) whose function is to extend the rangeof values of the colour intensities of a first interval [0:1] at asecond range of values [0:Lmax] where Lmax represents the length of thesecond interval, L_(max) being an integer greater than 1.

This ITMO module implements the method according to the invention whichwill be presented hereinafter with reference to FIG. 2. At the output ofthis module, the image sequence produced is in optical RGB format withintensities between 0 and L_(max).

Each image of the sequence is subjected in T3 to an inverse operation ofelectrical optical conversion so as to output a sequence of images whosecolour intensities correspond to a usable computer encoding for adisplay device, such as a TV set. For example, the conversionimplemented provides colour intensities in the Y′C_(b)C_(r) format whichdecomposes the colour intensities into a luminance component Y′separated from the chrominance components C_(b), C_(r). This formatY′C_(b)C_(r) is a way of representing the colour space in video that iswell suited to the transmission problematics. These components areencoded on 10 bits. As a variant, an additional conversion provides atT4 a sequence of output images in the format R′G′B′ encoded on at least10 bits.

The image sequence obtained is transmitted in T5 to a display device,such as for example a HDR digital television, for example, in accordancewith the ST2084 or STD-B67 standard.

In connection with FIG. 2, the steps of a method for processing adigital image according to an embodiment of the invention are described.

It is assumed that the optical colour intensities of the input image areexpressed in RGB format.

In a first step E0, the colour intensities of the input image areconverted into a colour space that comprises a luminance Y component andX and Z chrominance components. It is understood that in this space, aninformation representative of a luminosity level of the image at each ofits points is separated from the so-called chrominance information whichdefines its colour.

In E1, an information representative of a global luminosity level of theinput image, as perceived by the visual system of an observer, isdetermined.

According to a first embodiment of the invention, the determinedinformation is the key k of the image as defined by Masia.

According to a second embodiment of the invention, described withreference to FIG. 3, the global luminosity level information isdetermined in another manner, defined below:

In E11, the luminance component Y is converted into another luminancecomponent L*, so-called brightness component, of a colour space calledCIE L*a*b*. The brightness component L* can take values between 0(black) and 100 (white). This is a colour space for surface colours,defined by the International Commission on Illumination, (CIE) togetherwith the CIE L*u*v* colour space for light colours. Based on theevaluations of the CIE XYZ system, it was designed to more accuratelyreflect the differences in colours perceived by human vision.

In this model, three magnitudes characterise the colours, the brightnessL*, derived from the luminance (Y) of the XYZ evaluation, and twoparameters a* and b*, which express the difference in colour from thatof a grey surface of the same brightness, such as the chrominance of asequence of images.

As an alternative, this step may be skipped.

During a step E12, the median value L_(med)* of the brightness componentL* is calculated on all the elements of the input image IIn.

As an alternative, when the brightness L* is not obtained, a medianvalue Y_(med) is calculated from the luminance component values on allthe elements of the input image IIn.

As an alternative, a mean value of the brightness or the luminancecomponent value is obtained.

It is assumed that the image II_(n) has M image elements, with Mnon-zero integers.

For example, the median value is calculated by sorting the values of theluminosity components of the elements of the image in ascending order,the median value L_(med)* corresponding to the position (M+1)/2.

During a step E13, the median value obtained is normalised, so that itsvalue is between 0 and 1. We have:

$\begin{matrix}{L_{{med},n}^{*} = \frac{L_{med}^{*}}{100}} & (4)\end{matrix}$

As an alternative, the normalizing step is skipped for Y_(med), as thevalues of Y_(med) are already comprised in the following range [a,b] or]a,b] where a≥0 and b≤1. According to a preferred embodiment, a=0 andb=1.

In E14, the possible values for the median value of the normalisedbrightness are clipped, while excluding the extreme values of theinterval [0,1]. The new range of possible values is [0.05, 0.95].

Thus, we obtain an information representative of a global level ofluminosity of the input image equal to the normalised median value,which is clipped from the brightness component or the luminancecomponent value, depending on the concerned alternative:ILG= L _(med,n)* or ILG=Y _(med)  (5)

In relation to FIG. 2, the next step E2 of the processing methodaccording to the invention calculates an expansion coefficient γ as afunction of the information representative of a luminosity level of theILG input image. This expansion coefficient is intended to be applied tothe luminance component Y₁ of the input image IIn. According to theinvention, the expansion coefficient γ is calculated as a decreasingfunction of the ILG information.

According to a first embodiment of the invention, presented in relationto FIG. 4A, the expansion coefficient γ is calculated as a decreasingpolynomial function of the ILG information. For example, the expansioncoefficient γ is defined as follows:γ=α·ILG ² −β·ILG+ρ  (6)

with α=1.5, β=2.6 and ρ=2.2

According to a second embodiment of the invention, presented in relationto FIG. 4A, the expansion coefficient γ is calculated a logarithmicfunction of the inverse luminosity ILG information of the input image:

$\begin{matrix}{\gamma = {1 + {\log_{10}\frac{1}{ILG}}}} & (7)\end{matrix}$In these two examples, the ILG information is chosen equal toL_(med,n)*.

Of course, the invention is not limited to this particular case. Otherways of calculating the ILG information can be envisaged, for examplefrom the key of the image k.

An advantage of this function is that it does correspond to theperception model of the human visual system. In addition, it is simpleto calculate.

Of course, the invention is not limited to the use of these twoexamples. Other curves of models can be used.

Thus according to another embodiment of the invention, the expansioncoefficient γ is calculated from a logarithmic function of the ILGinformation, for example γ may be proportional to the inverse of thelogarithm of the ILG information of the input image:

$\begin{matrix}{\gamma = \frac{1}{{{gain} \times {\log_{10}\left( Y_{med} \right)}} + {offset}}} & \left( {7a} \right)\end{matrix}$where log₁₀ designates the decimal logarithm, γ the expansion exponentapplied to the first luminance component Y1, Y_(med) the medianluminance value which is normalised and clipped, gain a predeterminedgain value and offset a predetermined offset value.

This expression allows to obtain more suitable values of γ, when tonecurves are used for the step T1, allowing a better visual rendering.

For instance range of possible values for the gain is [0.01, 1].

For instance range of possible values for the offset is [0, 10].

Preferably, the value of γ is clipped at [c, d] where c≥1 and d≥2.

During a step E3, the luminance component Y₁ of the input image istransformed by applying the expansion coefficient γ:

$\begin{matrix}{{Y_{1}^{\prime} = Y_{1}^{\gamma}}{{{with}\mspace{14mu}\gamma} = {{1 + {\log_{10}\frac{1}{ILG}\mspace{11mu}{or}\mspace{14mu}\gamma}} = \frac{1}{{{gain} \times {\log_{10}\left( Y_{med} \right)}} + {offset}}}}} & (8)\end{matrix}$

and multiplying, for each element of the input image IIn, the luminancevalue Y by the amplitude of the luminance value interval s of thedisplay device L_(max).Y ₂ =L _(max) ·Y ₁′  (9)

For example, with an HDR screen standard such as ST 2084, if the maximumluminosity level of the screen is 1000 nits or cd/m², then L_(max) is1000.

In E4, the first chrominance components C1 of the image are transformedinto second components C2.

When the first components C1 are expressed in the form of three lightintensity values R1, G1, B1 of the RGB colour space, three secondcomponents R2, G2, B2 are obtained.

Several embodiments are considered.

FIG. 5 shows the steps of the method for processing an input image(II_(n)) according to the invention, when the input image is in standardSDR format and the display device is configured to render images(II_(n)) in HDR format.

According to a first embodiment, illustrated in FIG. 5, the firstchrominance components C1 are multiplied by an expansion coefficientproportional to the expansion applied to the luminance of the image, forexample equal to the Y₂/Y₁ ratio, as follows:

$\begin{matrix}{C_{2} = \frac{{C_{1} \cdot Y}\; 2}{Y_{1}}} & (10)\end{matrix}$

In the RGB colour space, we get:

$\begin{matrix}{R_{2} = \frac{{R_{1} \cdot Y}\; 2}{Y_{1}}} & (11) \\{G_{2} = \frac{{G_{1} \cdot Y}\; 2}{Y_{1}}} & (12) \\{B_{2} = \frac{{B_{1} \cdot Y}\; 2}{Y_{1}}} & (13)\end{matrix}$

An advantage of this mode is its simplicity.

An output image (IO_(n)) is thus obtained whose colour intensities takea wider range of values and adapted to the amplitude offered by thedisplay device.

To synthesise colours, a gamut or a colour gamut designate the portionof all the colours that a certain type of material, such as a TV screenor a computer monitor enable to render. The gamut depends on the primarycolours used to synthesise colours. It is often plotted on an area on achromaticity diagram by a polygon linking the points representative ofthese primaries. FIG. 6A shows the cloud of colour intensities taken bythe input image in the gamut according to the BT709 recommendationadapted to HDTV (for «High Definition TV»). FIG. 6B shows that of theoutput image obtained by the first embodiment of the invention which hasjust been described in relation to FIG. 5.

According to a second embodiment, illustrated in FIG. 5A, the step E4comprises a substep E41 for correcting the colour components, whichconsists in applying to the first chrominance components a correctionfunction that is no longer directly proportional to the Y2/Y1 ratiobetween output luminance and input luminance, as in the previousembodiment. According to this second embodiment, the correction functionthat applies to the first luminance components, depends on the first andsecond luminance components and a saturation factor s, which is a realnumber, preferably greater than 1, according to the followingexpression:

$\begin{matrix}{C_{2} = {\left( {{\left( {\frac{C_{1}}{Y_{1}} - 1} \right) \cdot s} + 1} \right)Y_{2}}} & (14)\end{matrix}$In the RGB colour space, we get:

$\begin{matrix}{R_{2} = {\left( {{\left( {\frac{R_{1}}{Y_{1}} - 1} \right) \cdot s} + 1} \right)Y_{2}}} & (15) \\{G_{2} = {\left( {{\left( {\frac{G_{1}}{Y_{1}} - 1} \right) \cdot s} + 1} \right)Y_{2}}} & (16) \\{B_{2} = {\left( {{\left( {\frac{B_{1}}{Y_{1}} - 1} \right) \cdot s} + 1} \right)Y_{2}}} & (17)\end{matrix}$

For example, the saturation factor s is chosen equal to 1.25.

An advantage of this correction is that by saturating the intensities ofthe colour components, it allows to obtain a more intense colourrendering.

Advantageously, the step E 4 further comprises a substep E42 forconverting the second chrominance components of a first colour space,larger than the first.

A conversion of a gamut A to a gamut B can be done by matrixtransformation as follows:

$\begin{matrix}{\begin{bmatrix}R_{2}^{\prime} \\G_{2}^{\prime} \\B_{2}^{\prime}\end{bmatrix}_{{Gamut}\mspace{11mu} B} = {\begin{bmatrix}a_{1} & a_{2} & a_{3} \\a_{4} & a_{5} & a_{6} \\a_{7} & a_{8} & a_{9}\end{bmatrix} \cdot \begin{bmatrix}R_{2} \\G_{2} \\B_{2}\end{bmatrix}_{{Gamut}\mspace{11mu} A}}} & (18)\end{matrix}$

For example, the intensities R2, G2, B2 obtained, which belong to afirst colour space, for example according to the BT709 recommendation,are converted into intensities R2′, G2′, B2′ in a second colour spacesuch as the new space according to the BT2020 recommendation recentlycreated for the new UHDTV screens (for “Ultra High DefinitionTelevision” set).

In this case, the conversion of the gamut according to the BT709recommendation to the gamut according to the BT2020 recommendation isdone by applying the following matrix, as specified in the BT2087recommendation:

$\begin{matrix}{\begin{bmatrix}R_{2}^{\prime} \\G_{2}^{\prime} \\B_{2}^{\prime}\end{bmatrix}_{{BT} \cdot 2020} = {\begin{bmatrix}0.6274 & 0.3293 & 0.0433 \\0.0691 & 0.9195 & 0.0114 \\0.0164 & 0.0880 & 0.8956\end{bmatrix} \cdot \begin{bmatrix}R_{2} \\G_{2} \\B_{2}\end{bmatrix}_{{BT} \cdot 709}}} & (19)\end{matrix}$

One advantage of this conversion is that it enables, due to increasedsizes of the polygon gamut, to ensure that the colour intensitiestransformed are located away from its borders in the second colourspace, which avoids the clipping effects of the colour intensities onthe output image.

FIG. 6C shows a chromaticity diagram of the output image obtained afterthe colour correction in step E41 according to the second embodiment. Itcan be noted that, due to the correction, the range of colourintensities is more extensive in FIG. 6C than in FIG. 6B, which isreflected, when returning the output image, by more intense rendering.

FIG. 6D shows a chromaticity diagram of the output image obtained afterthe step E42 of colour space change. It can be seen that the change incolour space makes it possible to move the cloud of intensity valuesaway from the boundaries of the gamut triangle, which has the effect ofavoiding any truncation of the colour intensities at the edges of thetriangle and therefore the appearance of rendering defects on the outputimage.

For a sequence of images, steps E1 to E4 are repeated for each image.

The invention which has just been presented has been tested on arepresentative set of image sequences belonging to diversified styles orlighting classes.

Lighting style designates the lighting and contrast conditions selectedby an artist to create an image, a photo or a video sequence. Theseconditions help to give the image a special atmosphere. The notion ofstyle is known and widely used in photography, television and cinema.The three following classes are identified in particular:

-   -   Medium-Key Lighting (MK) is a style of shooting images combining        a medium contrast to a moderate luminosity. Most of the image        and video content falls into this category.    -   Low-Key Lighting (LK) is a deliberately dark image taking style,        thus having a low luminosity associated with a high contrast.        The low key style is involved in several effects such as the        chiaroscuro. Contrary to standard lighting based on three light        sources, the Low-Key technique usually involves only one source.    -   High-Key lighting (HK) is an image-shooting style which combines        high luminosity with low contrast to express a sweet atmosphere.        It is a style widely used in the field of fashion and        advertising.

Taking into account the two dimensions given by luminance and contrast,the inventors offer a 2D classification comprising the following twoadditional styles:

-   -   Dark-Key Lighting (DK) is a deliberately underexposed        image-shooting style thus having a low luminosity associated        with a low contrast. It is particularly popular for night        scenes, by creating a disturbing atmosphere, raising the        suspense in horror movies or in thrillers;    -   Bright-Key Lighting (BK) is an image-shooting style that        combines a high level of contrast with high luminosity. This        style is commonly used for outdoor shooting on clear sunny days.

In relation to FIG. 7, the five lighting styles that have just beendescribed have been plotted in a diagram, based on informationrepresentative of global luminosity and contrast levels, as perceived byan observer. It can be seen that the classes LK and DK have comparableluminosity levels and are distinguished from each other by their levelof contrast. The same goes for classes HK and BK.

In connection with FIGS. 8A to 8E, 9 and 10, we now present the resultsobtained by the invention. FIGS. 8A to 8E compare for the five classesof images, the plots of the extended luminance values Y₂ of the outputimage according to those Y₁ of the input image, respectively obtained bythe invention and Akyuz's and Masia's methods, already described. As areminder, Akyuz's method uses an expansion exponent equal to 1 whileMasia's method exploits an exponent expressed as an affine function ofthe image key.

For the image of FIG. 8A, belonging to the Dark Key style class, theexpansion exponent γ calculated by the invention is 1.72 and thatcalculated by Masia is −0.35. With Akyuz's method, the expansion islinear, so that all luminances are increased in the same way.

The coefficient calculated by Masia is negative, which has the effect ofsaturating the luminance values Y₂. In relation to FIG. 9, whichpresents an image of the output sequence obtained by each of the methodsand the corresponding original image, as well as aesthetic and accuracyscores attributed by observers, it is checked that the image obtained byMasia's method is very white and has lost all contrast. The imageproduced by Akyuz's method is of good quality but the original lightingstyle has been distorted.

For the image of FIG. 8B, belonging to the Low Key style class, theexpansion exponent γ calculated by the invention is 1.5 and thatcalculated by Masia is −0.80. The same observations apply for the imagesproduced by Akyuz and Masia.

For the image of FIG. 8C, belonging to the Medium Key style class, theexpansion exponent γ calculated by the invention is 1.34 and thatcalculated by Masia is 0.39. The plot obtained by the method accordingto the invention lies below the Akyuz line. It therefore stretches theintermediate luminance values less than Akyuz does. This is verified inFIG. 8, which shows that the version of the image MK extended by theinvention globally looks less exposed than the version produced byAkyuz. The image produced by Masia is overexposed and the rendering isunnatural.

For the image of FIG. 8D, belonging to the Bright Key style class, theexpansion exponent γ calculated by the invention is 1.06 and thatcalculated by Masia is 1.7. The result obtained by Akyuz isover-illuminated. The one obtained by Masia is too contrasted. Inrelation to FIG. 8, the scores obtained by Masia are significantly worsethan those of Akyuz and those of the invention, notably because of aflickering between the images of the output sequence.

For the image of FIG. 8E, belonging to the High Key style class, theexpansion exponent γ calculated by the invention is 1.02 and thatcalculated by Masia is −2.65. The plot of the invention is very close tothe Akyuz line, but slightly below. FIG. 8 shows a more significantoverexposure of the version produced by Akyuz than that of theinvention. In relation with FIG. 9, the image produced by Masia is verywhite, devoid of any contrast.

In general, it can be noted that Masia's method tends to saturate theluminance values over the entire range of values taken by the inputimage, which will result in an impression of overexposure and loss ofcontrast.

Akyuz's method linearly amplifies luminance over the entire range ofvalues. The rendering of the images is acceptable, but the originalstyle of the images has been distorted.

These results emphasise the good results obtained by the invention whichfaithfully renders the lighting styles of the images processed by theinvention.

In relation to FIG. 10, we present the average scores assigned by a setof observers to the images produced by the three methods tested. Wedistinguish an aesthetic score, from a test without reference and anaccuracy score, from a test with reference. It can be seen that thescores obtained by the processing method according to the invention arealways higher than those of the other methods, whether from an aestheticor a fidelity point of view.

The man skilled in the art would be able to adapt the invention toconvert the HDR content received in SDR format before display. Accordingto this embodiment, the step E3 is logically reversed to get:

$\begin{matrix}{{Y_{1}^{\prime} = Y_{1}^{\frac{1}{\gamma}}}{{{with}\mspace{14mu}\gamma} = {{1 + {\log_{10}\frac{1}{ILG}\mspace{11mu}{or}\mspace{14mu}\gamma}} = \frac{1}{{{gain} \times {\log_{10}\left( Y_{med} \right)}} + {offset}}}}} & \left( {8a} \right)\end{matrix}$

and then

$\begin{matrix}{Y_{2} = {\frac{Y_{1}^{\prime}}{L_{\max}}.}} & \left( {9a} \right)\end{matrix}$

Moreover, the electrical optical conversion follow an inverse tone curvethan the one used for the conversion from SDR to HDR format.

It will be noted that the invention just described, can be implementedusing software and/or hardware components. In this context, the terms“module” and “entity” used in this document, can be either a softwarecomponent or a hardware component or even a set of hardware and/orsoftware, capable of implementing the function(s) outlined for themodule or entity concerned.

In relation to FIG. 11, we now present an example of simplifiedstructure of a device 100 for processing a digital image according tothe invention. The device 100 implements the processing method accordingto the invention which has just been described in connection with FIG.1.

This FIG. 11 illustrates only a particular way, among several possible,to fulfil the algorithm detailed above in relation to FIG. 2. Indeed,the technique of the invention is carried out indifferently on areprogrammable computing machine (a personal computer, a DSP processoror a microcontroller) executing a program comprising a sequence ofinstructions, or on a dedicated computing machine (for example a set oflogical gates such as an FPGA or an ASIC, or any other hardware module).

In the case where the invention is implemented on a reprogrammablecomputing machine, the corresponding program (that is to say thesequence of instructions) can be stored in a removable storage medium(such as for example a floppy disk, a CD-ROM or a DVD-ROM) or not, thisstorage medium being readable partially or totally by a computer or aprocessor.

For example, the device 100 comprises a processing unit 110, equippedwith a processor μ1 and driven by a computer program Pg1 120, stored ina memory 130 and implementing the method according to the invention.

At initialisation, the code instructions of the computer program Pg₁ 120are for example loaded into a RAM before being executed by the processorof the processing unit 110. The processor of the processing unit 110implements the steps of the method described above, according to theinstructions of the computer program 120.

In this exemplary embodiment of the invention, the device 100 comprisesa reprogrammable computing machine or a dedicated computing machine,capable of and configured for:

-   -   obtaining GET a input image II;    -   converting CONV the RGB colour intensities of the input image        into a colour space that comprises a luminance Y component and X        and Z chrominance components;    -   determining DET ILG an information representative of a global        luminosity level of the image, perceived by an observer, based        on the values of the first luminance component of the elements        of the image;    -   calculating CALC an expansion exponent γ as a function of the        global luminosity level information determined;    -   transforming TRANSF the first luminance components of the image        elements into second luminance components, comprising for an        element of the image, the calculation of an intermediate        luminance value by applying the expansion exponent calculated to        the first value of luminance component and the multiplication of        the calculated intermediate value by the length of the second        range of predetermined luminance values.

According to the invention, the calculated expansion exponent γ is adecreasing function of the determined global luminosity levelinformation.

Advantageously, the computing machine is configured to implement theembodiments of the invention which have just been described in relationto FIGS. 2 to 6.

In particular, it is configured to implement a transformation of thefirst chrominance components into second chrominance componentsaccording to the first or second embodiments described in relation toFIGS. 5 and 5A.

The device 100 further comprises a storage unit M₁ 140, such as a memoryor buffer, able to store, for example, the input image sequence, thecalculated expansion coefficient γ and the intermediate luminance valuesand/or the sequence of output images.

These units are controlled by the processor p1 of the processing unit110.

Advantageously, such a device 100 may be integrated in a user terminalequipment TU, for example a computer, a set-top box, a digitaltelevision set. The device 100 is then arranged to cooperate at leastwith the following module of the terminal TU:

-   -   a data transmission/reception module E/R, through which a signal        comprising encoded data representative of the input image        sequence is received from a telecommunications network, for        example a wired or hertzian radio network; and/or    -   an acquisition module of the input image sequence, such as for        example a video camera, for example via an HDMI cable.    -   a display device configured to reproduce images with a range of        extended colour intensities, such as a TV HDR professional type        Sony® BVM-X300 OLED equipped with SLoq3 and ST2084 transfer        functions. This device complies with BT.709 and BT.2020 colour        standards. It offers a maximum luminosity of 1000 nits.

Thanks to its good performance and its simplicity of implementation, theinvention which has just been described allows several uses. Its firstapplication is the conversion of video content in SDR format into aversion that can be displayed on an HDR rendering device. For example,it can be implemented live upon receipt of a video content in formatSDR, as a post treatment, for its display of the sequence of images on ascreen HDR.

For the live production of TV content using several acquisition modules,SDR and HDR, it can be used to convert SDR content into HDR on the flybefore mixing with HDR content. It can also prove interesting in filmpost-production.

Finally, the invention can be implemented at any point of a transmissionchain for transcoding a content transmitted in BT.709 HDR format into anHDR format, as specified by the ST2084 or STD-B67 standard.

An exemplary embodiment of the present invention improves the situationdiscussed above with respect to the prior art.

An exemplary embodiment of the invention in particular overcomes theseshortcomings of the prior art.

More precisely, an exemplary embodiment offers a solution thatguarantees a more realistic and more respectful rendering of theoriginal lighting style of the input image, while remaining simple toimplement and compatible with real-time constraints.

It goes without saying that the embodiments which have been describedabove have been given by way of purely indicative and non-limitingexample, and that many modifications can be easily made by those skilledin the art without departing from the scope of the invention.

The invention claimed is:
 1. A method comprising the following actsperformed by a processing device: receiving at least one digital image,said image comprising image elements, an image element being associatedwith colour information represented in a first colour space comprising afirst luminance component (Y₁), said first luminance component having afirst value within a first range of predetermined values; processing theat least one digital image for rendering on a display device, saiddisplay device being adapted to restore second values of secondluminance components of the image elements included in a second range ofpredetermined values, wherein the processing comprises: determining aninformation representative of a global luminosity level of the imageperceived by an observer, based on the first values of the firstluminance component of the elements of the image; calculating anexpansion exponent (γ) as a function of the global luminosity levelinformation determined; transforming the first luminance components ofthe image elements into said second luminance components, comprising foran element of the image, the calculation of an intermediate luminancevalue by applying the expansion exponent calculated to the first valueof the first luminance component, the multiplication of the calculatedintermediate value by a length of the second range of predeterminedluminance values, and obtaining the second value of the second luminancecomponent based on the multiplied intermediate luminance value, whereinthe calculated expansion exponent is a decreasing function of thedetermined global luminosity level information, based on the logarithmof said determined global luminosity level information.
 2. The methodaccording to claim 1, wherein the calculated exponent is proportional tothe inverse of the logarithm of the information representative of aglobal luminosity level of the image.
 3. The method according to claim2, wherein determining a global luminosity level comprises obtaining amedian value of the luminance component of the image, and wherein theinformation representative of a global luminosity level of the image isproportional to the median value obtained.
 4. The method according toclaim 3, wherein when the first range of predetermined values is oflength greater than the second range, the transforming act implementsthe following equations:$Y_{2} = \left( \frac{Y_{1}}{L_{\max}} \right)^{\frac{1}{\gamma}}$${{with}\mspace{14mu}\gamma} = \frac{1}{{{gain} \times {\log_{10}\left( Y_{med} \right)}} + {offset}}$where Y1 designates the first luminance component, Y2 the secondluminance component, L_(max) the length of the second range of luminancevalues, log₁₀ the decimal logarithm, γ the expansion exponent applied tothe first luminance component Y1, Y_(med) the median luminance value,gain a predetermined gain value and offset a predetermined offset value.5. The method according to claim 3, wherein when the second range ofpredetermined values is of length greater than the first range, thetransforming act implements the following equations:Y₂ = L_(max) ⋅ Y₁^(γ)${{with}\mspace{14mu}\gamma} = \frac{1}{{{gain} \times {\log_{10}\left( Y_{med} \right)}} + {offset}}$where Y1 designates the first luminance component, Y2 the secondluminance component, L_(max) the length of the second range of luminancevalues, log₁₀ the decimal logarithm, γ the expansion exponent applied tothe first luminance component Y1, Y_(med) the median luminance value,gain a predetermined gain value and offset a predetermined offset value.6. The method according to claim 1, wherein the first colour spacefurther comprising first chrominance components, said method furthercomprising transforming the first chrominance components (C1) of theimage into second chrominance components (C2), by applying to the firstchrominance components an expansion coefficient proportional to a ratiobetween the second luminance component and the first luminancecomponent, according to the following expression:$C_{2} = {\frac{{C_{1} \cdot Y}\; 2}{Y_{1}}.}$
 7. The method forprocessing at least one digital image according to claim 1, wherein thefirst colour space further comprising first chrominance components, saidmethod further comprising transforming the first chrominance components(C1) of the image into second chrominance components (C2), comprising asubact of colour correction by applying to the first chrominancecomponents a correction function which depends on the first and thesecond luminance components (Y1, Y2) and a saturation factor (s), whichis a real number having a predetermined value, according to thefollowing expression:$C_{2} = {\left( {{\left( {\frac{C_{1}}{Y_{1}} - 1} \right) \cdot s} + 1} \right){Y_{2}.}}$8. The method for processing at least one digital image according toclaim 1, further comprising prior to the transforming act, setting theexpansion exponent to a first predetermined value if the calculatedvalue is less than said first predetermined value or setting theexpansion component to a second predetermined value if the calculatedvalue is greater than said second predetermined value.
 9. The method forprocessing at least one digital image according to claim 7, wherein theact of transforming the first chrominance components comprises a sub-actof converting second chrominance components from a first colour space toa second colour space, larger than the former.
 10. A device comprising:a reprogrammable computing machine or a dedicated computing machine,capable of and configured to: receive at least one digital image, saidimage comprising image elements, an image element being associated withcolour information represented in a first colour space comprising afirst luminance component separated from chrominance components, saidfirst luminance component having a first value comprised in a firstpredetermined value range; and process the at least one digital imagefor rendering the image on a display device, said display device beingadapted to render second values of second luminance component of theimages elements included in a second predetermined range of values,wherein the processing comprises: determining an informationrepresentative of a global luminosity level of the image, perceived byan observer, based on the values of the first luminance component of theelements of the image; calculating an expansion exponent as a functionof the global luminosity level information determined; transforming thefirst luminance components of the image elements into said secondluminance components, comprising for an element of the image, thecalculation of an intermediate luminance value by applying the expansionexponent calculated to the first value of the first luminance component,the multiplication of the intermediate value calculated by a length ofthe second range of predetermined luminance values, and obtaining thesecond value of the second luminance component based on the multipliedintermediate luminance value, wherein the calculated expansion exponentis a decreasing function of the determined global luminosity levelinformation, based on the logarithm of said determined global luminositylevel information.
 11. A terminal equipment capable of and configured toobtain a sequence of digital images and to transmit a sequence ofdigital images to the display device, wherein the terminal equipmentcomprises the device according to claim
 10. 12. A non-transitorycomputer-readable storage medium, storing a computer program productcomprising instructions, which when executed by a reprogrammablecomputing machine configure the computing machine to perform actscomprising: receiving at least one digital image, said image comprisingimage elements, an image element being associated with colourinformation represented in a first colour space comprising a firstluminance component (Y₁) and first chrominance components, said firstluminance component having a first value within a first range ofpredetermined values; processing the at least one digital image forrendering on a display device, said display device being adapted torestore second values of second luminance component of the imageelements included in a second range of predetermined values, wherein theprocessing comprises: determining an information representative of aglobal luminosity level of the image perceived by an observer, based onthe values of the first luminance component of the elements of theimage; calculating an expansion exponent (γ) as a function of the globalluminosity level information determined; transforming the firstluminance components of the image elements into said second luminancecomponents, comprising for an element of the image, the calculation ofan intermediate luminance value by applying the expansion exponentcalculated to the first value of first luminance component, themultiplication of the calculated intermediate value by a length of thesecond range of predetermined luminance values, and obtaining the secondvalue of the second luminance component based on the multipliedintermediate luminance value, wherein the calculated expansion exponentis a decreasing function of the determined global luminosity levelinformation, based on the logarithm of said determined global luminositylevel information.